Actually it can be rectified quite easily but the danger is making things worse. Say left chainstay is 1/8" longer than the right so the dropout sits close to an 1/8" lower. You could file a little less than 1/8" off the top of the left dropout, doing FB's checks as you go. Of course,if you fil so much off the dropout bends or breaks, you went too far. With brazing skills, additional material could be added for strength at the expense of paint.

I've never done this. I'd listen to those who have before trying. But I've heard of it being done. If this mixte is of the UO-8 variety with the plate steel cut dropout, you are hardly going to cause any issues until you weaken it until it bends and even that won't be catastrophic. (Says he who rode his UO-8 into the ground (and two car doors - crashing perhaps 2 to 4 dozen times on it - 5 crashes a winter was the norm - and bent his right dropout beyond the ability to run a derailleur on it early on. Mild steel, Very forgiving. At 19,000 miles the right chainstay broke. I rode it home and had a friend weld it. 22,000 miles was the second car door, trashed fork and I didn't need the reminder so it got retired.)